1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus, more specifically to an optical scanning device which scans scan target surfaces with light and an image forming apparatus including the optical scanning device.
2. Description of the Related Art
In the field of electrophotographic image recording, an image forming apparatus using a laser is widely used. The image forming apparatus includes, for example, a photosensitive drum (hereinafter also referred to as the photoconductor drum) and an optical scanning device which forms a latent image on an outer circumferential surface of the photoconductor drum. The optical scanning device includes, for example, a light source which emits laser light, an optical deflector, such as a polygon mirror, for example, which deflects the laser light emitted from the light source, and a scanning optical system which detects the laser light deflected by the optical deflector onto the outer circumferential surface of the photoconductor drum.
In recent years, along with the colorization of images and the increase in processing speed of image forming apparatuses, a tandem-type image forming apparatus including a plurality of photoconductor drums (normally four) has been increasingly used.
The tandem-type image forming apparatus tends to be increased in size in accordance with the increase in the number of photoconductor drums. Therefore, there is a demand for a reduction in device size, including a reduction in size of the optical scanning device. To reduce the size of the optical scanning device, it is effective to cause optical paths of a plurality of laser lights directed from the optical deflector toward the respective photoconductor drums to overlap one another.
For example, according to a background optical scanning device, a first light beam and a second light beam perpendicular to each other in polarization direction are deflected by a deflector and guided to and imaged on respective scan target surfaces via a scanning optical system. In the background optical scanning device, the scanning optical system includes a first scanning lens, a second scanning lens, and a polarization separation element. The first scanning lens is made of glass and disposed on the optical paths of the light beams deflected by the deflector. The second scanning lens is made of resin and disposed on the optical paths of the light beams transmitted through the first scanning lens. The polarization separation element is disposed on the optical paths of the light beams transmitted through the second scanning lens. The polarization separation element transmits therethrough the first light beam, and reflects the second light beam.
It is assumed in the background optical scanning device that the respective polarization directions of the two light beams incident on the polarization separation element are perpendicular to each other. The perpendicular relationship between the polarization directions of the two light beams, however, may fail to be maintained before the light beams are incident on the polarization separation element owing to such factors as installation error of a light source and birefringence occurring in the scanning lens molded of resin.
In this case, the polarization separation element, which is intended to completely reflect one of the light beams, instead transmits therethrough a portion of the light beam. As a result, the transmitted portion of the light beam reaches, as a ghost light, a scan target surface which is not intended to be reached by the light beam. At the same time, the polarization separation element, which is intended to completely transmit therethrough the other one of the light beams, instead reflects a portion of the light beam. As a result, the reflected portion of the light beam reaches, as a ghost light, a scan target surface which is not intended to be reached by the light beam. These ghost lights degrade the quality of the image output from the image forming apparatus.
According to the background optical scanning device, it is difficult to remove the ghost lights.